Effect of Water, Oxygen, and Air Exposure on CH3NH3PbI3-xClx Perovskite Surface Electronic Properties

The stability of hybrid perovskite materials for solar cells and high surface property reproducibility are important topics of present research. To understand the impact of water, oxygen, and air exposure, as experienced during typical device fabrication and operation conditions, on the surface electronic properties of solution-processed CH3NH3PbI3-xClx perovskite thin films, and subsequently formed interfaces to other materials, a comprehensive photoelectron spectroscopy study is performed. It is shown that pure oxygen exposure reduces the typically observed pronounced n-type surface character, while pure water exposure increases it. Very low water partial pressure, e.g., as encountered in inert-gas glove boxes and in high vacuum of 10(-6) mbar, reduces the perovskite work function due to water physisorption, which is fully reversible upon mild annealing (room temperature to 50 degrees C) in ultrahigh vacuum. Upon exposure to ambient air, the effect of oxygen prevails over that of water, and the perovskite become less n-type as seen by a 0.6 eV Fermi level shift toward midgap. The results help in understanding variations of the surface electronic properties reported for perovskites, which in turn impact the energy level alignment at heterojunctions in devices. For future studies, the crucial importance of stringent environment control is stressed upon.